Many years ago when first I was privy to see an actual platinum casting, I was amazed at the meticulous effort, the time and the skill that was required to make such a casting. The model was chosen, a small tree was waxed up, and the whole contraption was set on a thick paper and attached to the bottom of the flask.  It was then filled with the investment which was created by mixing a binder with the water in the proper proportion and set to dry after all the bubbles were removed.  After a long period of time, the flask then was placed in the kiln and slowly stepped up in temperature over an eight-hour period. The next morning, we cast the pieces in the flask, using a torch.  At de—vesting, the caster placed pieces in a hydrofluoric acid bath for cleaning.

It was quite an effort and dangerous too.  With today's modern induction machines and great casting alloys, casting platinum has become somewhat less of a challenge, but the traditional investments are still very demanding and require absolute process control.  After a long search in other industries using other technologies, I discovered in the dental industry a concept which I think will revolutionize the jewelry industry.

The Concept
Imagine an investment powder you could mix, with pre—packaged portions of powder to liquid so you won't make a mistake, that will set totally and completely in 15 minutes, that is so strong that it required no steel flask but a flexible plastic sleeve that is removed after the mass has set. And imagine further, that this material can be placed into a kiln, at 1700ºF for rapid burn—out to cast 20—30 minutes thereafter. And imagine that after the casting is done, the de—vesting would be easy, using water or sand blasting, and the metal would have a shiny surface, without the requirements of hydrofluoric acid for cleaning.  The material I am about to show you has all these properties.

Bi—metal Casting
The principle of bi—metal casting is really very simple: A wax model is invested, cast and finished.  A secondary wax model is created which is going to become part of this piece.  The wax is attached to the first casting, re—invested and the metal is cast on, thus creating a piece that is made with two metals.  It is recommended that mechanical fastening devices, such as plugs, tracks, etc. be worked into the design.  The piece is then finished as usual.

The Project
The flask system used for this project is a 2"x1 3/4" clear plastic flask, designed to hold one ring.  For this experiment, we are using a rapid investing dental investment system as is. The platinum section has been finished and completely polished. We are using Pt 900 with an iridium alloy, and the attached wax will be cast with 18K yellow gold.

Using the rapid dental investment and the required expansion fluid, we mix the prepackaged portions together.  This fills the flask exactly. The mix is vacuumed in the bowl for 60 seconds and in the flask for an additional 60 seconds.  The filled flask is then left to set for 15 minutes. During the setting or curing process, the chemical reaction will heat the flask to a temperature of 50ºC, which will soften the wax. After setting up, we gently remove the flask from the investment.  It will stand alone.

The Burn—out
Without a metal flask or even a plastic flask surrounding it, the dry and set investment in the flask shape is now placed in a kiln, preheated to 1700ºF. The burn—out time is determined by the cubic inches of investment present, and in this case is about 30 minutes.  During that time the wax will be completely gone, the investment will be bright red in appearance, and very hard.  At this point we simply turn the kiln off and let the temperature drop to a casting temperature level for 18K gold.  We found that 1000ºF works best.  The cooling takes about 15 minutes and with a total time of one hour since we filled the flask with investment, we are ready for casting.  We place the red flask into the casting machine, lock it in and melt the 18K gold with the torch and spin.

Cleaning and Casting
The button shows us that the casting was good. De—vesting was surprisingly easy.  The investment held up to its promise of being stronger than most anything on the market.  The bond the 18K made on the platinum was good, strong and seamless.

Finishing
Sawing off the button, we file, sand and polish the gold portion of the ring.  A final buffing colors the platinum, and the ring is finished.

Summary
Bi—metal casting is a wonderful way to blend two metals together.  Many tricks can be used to accomplish a good bond at the interface. Solder applied as a thin film to the platinum surface can be helpful.  Holes drilled into the host metal will act as lugs; or small pegs can be attached as we did in this example. The rapid investment method demonstrated made it possible to do the entire bi—metal casting in under two hours, from a wax attached to a platinum host ring to a showroom piece ready for delivery.

I believe that the newly developed rapid investment powders will make a big difference in the way castings are being done, be it on a small scale or a volume manufacturing level.  An investment that can cast platinum in under two hours, with a bright shiny surface, eliminating the need for hydrofluoric acid, that has the capability of being used in a vacuum casting machine, is something that until now we could only dream about. Currently, I am researching the possibility to modify existing rapid investment powders to be adapted to casting platinum. By adding a slurry to the wax before investing, a smooth surface can be accomplished.  The investment powder that we are describing will work for all precious metal and has vacuum cast capability.
 

V4N3

Casting Gold to Platinum
Jurgen J. Maerz
Manager of Technical Education
Platinum Guild International
Ricardo Basta/Eichberg
E. Eichberg Jewelers

This is an abbreviated version of the original work. For full technical details, please consult the original paper.